Pigment compositions use pigments having excellent light fastness as coloring materials. Utilization of the excellent light fastness of the pigments allows the pigment compositions to be used in various indoor and outdoor applications such as the field of coatings for automobiles and buildings, the field of printing inks such as offset inks, gravure inks, flexographic inks, and silk screen inks, and the field of aqueous inks for inkjet recording.
In particular, aqueous inks that use water as their main solvent have an advantageous feature in that the risk of fire, which is high for solvent inks, can be further reduced. The aqueous inks are the mainstream of inks particularly for inkjet recording.
A pigment is insoluble in water and is used for an aqueous pigment dispersion prepared by dispersing the pigment in an aqueous medium. Accordingly, methods for stably dispersing a pigment in an aqueous medium have been studied.
For example, in one known method, an (An-Bm) block copolymer is used as a pigment dispersant. The (An-Bm) block copolymer is an A-B block copolymer, wherein A is styrene, and B is acrylic acid. The degree of polymerization of A is about 5 to about 50, and the degree of polymerization of B is about 70 to about 800 (see, for example, PTL 1).
One challenge of an aqueous pigment dispersion is to reduce the number of coarse particles that may be formed during dispersion. The coarse particles are particles having diameters much larger than the average particle diameter of the aqueous pigment dispersion obtained and may be aggregates of undispersed particles of the pigment and debris of the pigment formed during dispersion or aggregates of the polymer used as a pigment dispersant.
The coarse particles inhibit the formation of a uniform coating surface during coating or printing. In a printing method in which an ink such as an aqueous ink for inkjet recording is ejected from nozzles of an inkjet head, the coarse particles cause clogging of the nozzles. In particular, since the resolution of inkjet printers is increasing in recent years, the density of nozzles of inkjet heads is increasing, and the size of liquid droplets is decreasing. In other words, the diameter of the nozzles for ejecting the ink is decreasing, and the degree of integration of the nozzles is increasing (see, for example, PTL 2). As the diameter of the nozzles decreases, the allowable size of foreign matter decreases, and this causes an increase in the frequency of occurrence of clogging of the nozzles. Specifically, a problem occurs in that inks applicable to conventional inkjet printers cannot be used for a printer equipped with a newly developed high-resolution inkjet head.
In order to provide an ink that can cope with a reduction in the diameter of nozzles and an increase in their degree of integration in recent years, there is still room for consideration. In view of the above, the present applicant has developed and disclosed an aqueous pigment dispersion used to provide an aqueous ink for inkjet recording that contains a reduced number of coarse particles, is excellent in ejection stability, and is applicable to a printer equipped with a high-resolution inkjet head (see PTL 3).
Aqueous inks for inkjet recording are used as basic color inks including yellow, magenta, cyan, and black inks, and these basic color inks are often used in combination with, for example, an aqueous green, red, blue, or orange ink called an extra color ink. Known examples of a green pigment include polyhalogenated metal phthalocyanines such as C.I. Pigment Green 36 and C.I. Pigment Green 7 (see, for example, PTL 4). Known examples of red and orange pigments include diketopyrrolopyrrole pigments (see, for example, PTL 5 to PTL 7).
However, the polyhalogenated metal phthalocyanine pigments and the diketopyrrolopyrrole pigments have larger specific gravities and larger primary particle diameters than pigments used for the basic colors such as cyan and therefore tend to sediment, so that it is sometimes difficult to obtain dispersion stability comparable to that of the basic color pigment dispersions.
The dispersion stability of a pigment often depends on the interaction between the type of the pigment and a dispersant resin. Therefore, even when a dispersant resin used for a basic color pigment dispersion is used in combination with an extra color pigment, it is not always possible to readily obtain good dispersion stability. In order to improve the dispersion stability of extra color pigments, those skilled in the art may need considerable trial and error.